TECHNICAL FIELD
1. Field of the Invention
The present invention relates to a control method for a servomotor for driving a feed shaft of a machine tool or a robot arm, and more particularly, to a feedforward control method for a servomotor.
2. Description of the Related Art
In controlling a feed shaft of a machine tool or a robot arm by means of a servomotor, feedforward control is used in order to reduce the position deviation. When using the machine tool for high-speed cutting, in particular, shape errors are liable to be caused by a follow-up delay of a servo system. Thus, a position loop is feedforward-controlled to reduce these errors.
In the conventional feedforward control, variation of a move command for each position and speed processing period is differentiated, and a value obtained by multiplying the resulting differential value by a feedforward coefficient is added to a speed command obtained in a conventional position loop processing, thereby correcting the speed command. A speed loop processing is executed in accordance with the corrected speed command to reduce the position deviation, thereby correcting the servo delay.
However, a distribution period (ITP period) during which the move command is delivered from a numerical control device to a servo circuit side, that is, during a position loop, is about 8 msec, yet the period to execute a position/speed loop process in the servo circuit is only 2 or 1 msec. The position loop is therefore controlled so that the ITP period, during which the move command is delivered from the numerical control device, is divided into position loop periods, Ts and move commands for the individual divided position loop periods are equal. Thus, even though an acceleration/deceleration time constant is given to the move command outputted from the numerical control device, the position loop is controlled so that the move commands for individual position loop processing periods Tp in each ITP period are equal. Accordingly, a great difference is produced between the move commands for the position loop processing periods at the turning of the ITP periods, and this is differentiated into a great value by means of a feedforward term. Thus, the speed command contains a high-frequency component, which cannot be followed up in a speed loop. As a result, the position deviation is subject to variation, causing a great shock on the action of the motor or machine.
In order to eliminate this drawback, the inventor hereof filed an international application (PCT/JP90/00380) for the following invention, which was internationally published as International Publication No. WO90/11562. According to this invention, an acceleration/deceleration processing is inserted in a feedforward term for speed control, as well as for position control, and a smoothing processing is executed to remove the variation. In this smoothing operation, past data are leveled, and a time delay is equivalently caused in the feedforward term.